Minimum Quantity Lubrication Systems (MQL) – How to reduce energy and consumable costs without comprising machining performance.

MQL as an acronym (Minimum Quantity Lubrication) is becoming slowly more understood in the machine tool industry as its clear technical and cost benefits start to be delivered for users looking to reduce consumption costs and improve technical and environmental factors for their machining processes. Previously MQL had limited applications and was frequently associated with specialist processes such as turning or specialist mould and die finishing with ball end-mill applications. This article aims to explain the basic principles of the system and offers some recommendations on specific savings which may be achieved by users of MQL compared to conventional coolant systems. MQL utilises a soluble and/or bio-degradable cutting oil delivered in a fine mist to the machining area via either externally delivered nozzles or via integrated tooling to the work piece. It is used as an alternative to either dry machining where tool life is reduced or in place of high pressure flood coolant. Firstly, the disadvantages of flood coolant are numerous such as the high running cost of coolant pumps, maintenance of coolant systems, cleaning of the work area of the machine tool and particularly in summer or warm conditions the unpleasant odour sometimes emitted by poorly maintained coolant systems. It is probably a common viewpoint in the machining industry that a frequent observation of machine shops using flood coolant require a high level of maintenance not only in monitoring coolant condition but also clearing up spills from leaks and so on that present a health and safety hazard in the work place. While therefore it would be ideal to implement dry machining the effect on tool life, heat management and process performance means that this is not possible in most cases. Where dry machining is successfully employed, the process performance can be inhibited by the need to balance productivity against surface finish and tool life and maintenance costs. Therefore even where users have a defined dry cutting process, MQL can potentially assist with extending tool life and increase machining performance in terms of surface finish, cutting forces and process capability. To analyse the cost benefits of MQL, it is firstly worthwhile to study the typical categories of costs associated with the running of a conventional machine tool employing flood coolant in a production environment. When most people are asked about their opinion on which element contributes to the most electricity consumption of a typical CNC machine tool, most people may guess that it is either the main machine spindle (Either work holding or tool holding) or an axis servo motor. In reality these elements are much less, according to a study made by Kuroda shown in Figure 1, which identifies for mid-sized CNC machines that the main spindle only consumes about 12% of the total power consumption and around 7% for main axes feed from the servo motors. Comparatively however, the coolant system including chiller, pumps, and so on can contribute over 60% of the total machine power consumption costs. While this figure sounds relatively high, consider that in reality the machine axes are moving not all the time particularly if point to point positioning is being employed and modern servo motor technology actually produces very efficient motors from a power consumption perspective when not under load. Consider also that a spindle will not always be active either only during actual cutting operations ideally. Therefore a coolant pumping system with associated chillers needs to be ran fairly consistently despite the machine not actually cutting or machining and perhaps also off shift if coolant temperature needs to be maintained for optimum machining performance during actual shift hours to reduce idling time. Imagine thenn also larger users of CNC machine tool maybe also with centralised cooling system and you can start to visualise the potential cost implications of flood coolant systems in the overall consumption costs of machining operations.

Figure 1 – Kuroda study regarding the typical distribution of power consumption in a CNC machining system Further investigation by Kuroda also details the cost implications of coolant in the overall product costs associated with machining operations shown in figure 2 below which indicates that an average 16% of product cost from machining operations could be attributed to coolant systems and cutting tools may contribute 4%.

Figure 2 – Analysis of typical costs associated with machining processes It can be concluded that any improvement on consumption costs for machining operations either in power, tooling or maintenance downtime has to have a direct implication for not only revenue costs but direct product costs as well. Based on field study conditions, Kuroda were able to successfully analyse cutting conditions and calculate coolant consumption costs with their customer base. Shown in figure 3 and based on trials conducted with 4 flute 18mm end mills with depths of cut up to 15mm in carbon steel they recorded that the customer was able to reduce consumption of coolant from 18 litres per month to just 0.6 litre per month of soluble oil and from a cost perspective this represented a cost saving on coolant of 54% based on previous consumption.

Figure 3 – Consumption and costs of MQL versus traditional coolant The principle behind the generation of oil mist is well known from engineering principles utilising the Venturi effect. Via a supply of high pressure air flow generally present in most engineering work shops to a regulator on the machine tool, a chamber of soluble oil is pressurised whereby an oil mist is created by the oil being drawn up through a tube from the incoming pressure. As it passes back into the main tank, it is atomised into a mist from a spray nozzle. Heavier droplets will return back into the main tank liquid and lighter atomised droplets or mist will be carried via a port to the delivery point in the machine tool system from the system pressure.

Figure 4 – Principles of an oil/air mist system As the air mist is delivered along the pipes to the work area the ability to form and retain droplets which will stick to the target area is critically linked to flow speed of the air pressure within the lines as well as the diameter of the droplet itself which is defined by the nozzle exit profile and port design to ensure that the droplet formation is reduced to as small a diameter as possible and where droplet exit should ideally be close to the work piece surface.

Figure 5 – Principles of oil mist deposition While uniform droplet diameter distribution will naturally vary within defined limits, the majority of droplet formation will be 1 micron or less when using a tool integrated nozzle option. Another critical parameter is the choice of oil itself which is recommended as a bio-degradable oil with a specific density of 0.95 g/cm3 at 16 Deg C and importantly a kinematic viscosity of approximately 19mm2/s at 40 Deg C. To compare MQL with either dry machining, flood coolant or even air cooling it is necessary to study a number of factors including cutting force on the tool, tool wear, temperature and surface finish. Based on field case studies by Kuroda using end milling, they were able to demonstrate using the MQL process comparable cutting forces to flood coolant with comparable wear rates. Comparison of MQL samples at varying ml/hr delivery rates consistently reduced cutting force on the tool compared to dry machining with air cooling only. At higher rates of 16ml/hr, MQL was able to match the cutting force measured of flood coolant systems which was operating a much higher consumption rate of 258l/hr.

Figure 6 – Comparison of MQL, Coolant and air blow cooling The thermal effect of dry machining can also be compared and contrasted by the application of a thermal camera and temperature probe and based on extensive continuous cutting trials of 12m of continuous machining showed that with MQL the tool reached its working temperature of approximately 240 Deg C at the tool during cutting and thereafter proved very stable as opposed to a continuous growth in temperature profile of dry or air cooled machining.

Figure 7 – Thermal effects of MQL and non-coolant methods While cutting force was comparable with flood coolant and temperature was considered within acceptable limits and stable with the MQL process, tool wear is also an important factor in the cost analysis and process stability. After 12m of machining, flute edge quality was analysed and photographed and showed comparable and low wear conditions the same as flood coolant.

Figure 8 – Effects of cooling method on tool wear Surface finish is the final element studied by Kuroda and this can be influenced by a variety of factors such as the tool itself, material, feed rates and so on. In a separate test conducted by Kuroda, it was established that MQL could achieve a better surface finish for turning of special materials such as Titanium whereby a 60mm blank size was turned with 3 separate diameters and flood coolant versus 3 separate MQL mist pressures. The startling results indicated that the surface finish measured in Ra was actually lower than the performance achieved by flood coolant.

Figure 9 – MQL trials on surface finish machining Titanium To date, the rate of adoption is fairly low but is increasingly rapidly. Historically, the usage of MQL has been for limited applications such as deep hole drilling, mould and die milling and also some applications where coolant systems are not practical such as in robot based drilling of large components. In reality, the benefits of MQL can be applied to many differing industry types and chip forming processes to great effect. Modern MQL systems such as those provided by Kuroda can be directly fitted to CNC machine as a standalone system whereby they can be operated independently of the CNC system and as such can be purchased and installed by the end user of a machine tool without complex machine modification. Ideally however, the system can be incorporated into a CNC system under M Code actuation which requires more specialist integration either via an independent company or via the machine tool OEM. Kuroda Jena Tec principally offer three models of Eco Saver MQL system where two of the systems (KEP4 & KEP-V) can be offered as an integrated CNC version offering separate tank, delivery rate and pressure ranges and the third is a standalone system (KEP-WR) for use with retrofit by end users or manual machines The MQL process is not a universal solution for all machining requirements and does have limitations dependant on the application, material and geometry of the workpiece. For detailed assessment of the application, Kuroda Jena Tec have application engineers available to assist with determining the correct application of MQL and supply of equipment, process, installation and process development.